1. Field of the Invention
This invention pertains to arc welding, and more particularly to apparatus incorporated into the welding gun of wire welding machines.
2. Description of the Prior Art
In metal inert gas (mig) welding, the gun contact tip must satisfactorily perform two important functions. First, it must accurately guide the weld wire to the point of contact with the workpiece. Second, it must conduct adequate current, which can be as high as 400 amps and more, to the weld wire. To achieve those results, it is well known to provide a relatively long contact tip made of copper and having an axial hole therethrough. The hole is sized slightly larger than the diameter of the weld wire. The contact tip extends from a gas diffuser through which an inert gas is directed around the welding arc. Current to the contact tip is supplied to it through the gas diffuser.
Contact tips are particularly subject to at least two modes of failure. The first is burnback, which occurs if the feeding of weld wire through the contact tip is stopped while an arc is maintained. The second mode of failure is wearing of the through hole due to the abrasive nature of the weld wire material. In addition, because the contact tip is so close to the welding arc, and because the tip must carry high electrical currents, it becomes very hot. The high temperatures also have a deleterious effect on the contact tip, due in part because the hotter the tip becomes the softer it becomes, which makes it less resistant to wear from the weld wire.
Due to the severe operating conditions, contact tips have relatively short service lives. Consequently, they are treated as consumable items in the industry. Typically, contact tips are made as separate pieces that are threaded into the gas diffusers. When a contact tip has worn, it is threaded out of the gas diffuser and discarded, and a new one is screwed in place.
As noted above, in addition to the heat caused by the welding arc, another potential source of heat to the contact tip are the threads between it and the gas diffuser. That is, the threads can cause resistance heating. To minimize electrical resistance, the contact tip and gas diffuser threads have a relatively long engagement length and a fine pitch. A typical thread for prior contact tips and gas diffusers is 0.250-28 UNF with approximately 0.25 inches of axial engagement between the two parts.
However, the long engagement fine pitch thread presents a problem. Removing a worn contact tip from the gas diffuser requires several turns of the contact tip. For example, with 28 pitch threads, seven turns are necessary to produce 0.25 inches of axial movement for removal. If the weld wire has burned back and fused to the contact tip, the weld wire must twist through the same number of turns as the contact tip. The potential springback of the fully removed contact tip due to the twisted weld wire is annoying if not potentially dangerous. Moreover, the time required to disassemble the multiple turns also contribute to down time of the production area.
U.S. Pat. Nos. 5,338,917 and 5,440,100 show a welding gun in which a gas diffuser has a projection for contacting a channel of varying width in a contact tip. When installed together, the contact tip and gas diffuser contact each other at one point and along two lines. The total area of contact at the point and along the lines is quite small, thereby increasing the resistance heating produced by the welding current.
U.S. Pat. No. 2,761,049 shows a welding gun having a contactor tip received in and held by a cap nut to an elongated guide tube. There is a frusto-conical mating surface between the contactor tip and the guide tube. The contactor tip and cap nut are surrounded by a gas cup that is slidable along the guide tube. Replacing the contactor tip requires sliding the gas cup and turning the cap nut several turns both to remove the contactor tip and to install a new one. Because the cap nut is a separately removable piece, it is subject to loss or damage.
U.S. Pat. No. 2,808,498 shows an electrode contact tip buried deep in a welding gun. The weld wire emerges from the welding gun through a ceramic nozzle tip that is threaded into a water jacket. Replacement of the contact tip requires major disassembly and reassembly of the welding gun.
Thus, a need exists for improvements to the contact tips and gas diffusers of wire welding machines.